A conventional X-ray tube of the rotating anode type comprises a vacuum envelope having transversely disposed therein an anode target disk having a radially sloped surface with a focal spot area disposed in spaced alignment with an electron emitting cathode. A beam of electrons from the cathode being focused onto the aligned focal spot area of the target strikes the focal spot area and generates X-rays. The kinetic energy of the electrons bombarding the focal spot area of the target is partially converted into heat. In order to distribute the thermal load, the target disk is rotated at approximately 3,000 to 10,000 rpm. The target disk is mounted on a rotatable shaft which is coupled to the vacuum tube via bearings. The use of such X-ray tubes for medical diagnostics is quite common.
The rotation of the rotor anode structure generates vibration initiated by high rotation speeds and accelerations to which the rotor anode structure is exposed. A portion of this vibration ultimately is transmitted through the X-ray tube housing surrounding the vacuum tube. The space between a vacuum envelope and the housing is filled with oil which is used for X-ray vacuum tube heat dissipation and for dielectric purposes. When this vibration reaches the housing, it is radiated as noise. This acoustic noise has become a great concern to medical staffs and patients.
A number of the attempts have been made to reduce the vibration which leads to the noise. Attempts to reduce noise such as lowering the rotation speed of the target resulted in shortened life expectancy of the target. Certain efforts have been made to reduce X-ray tube noise without shortening target life expectancy by adding a mass on an anode shank mounting area. U.S. Pat. No. 4,935,948 "X-Ray Tube Noise Reduction by Mounting a Ring Mass" discloses attaching a ring mass on or near the bearing shroud which physically connects the rotor bearings to the vacuum tube. Such a ring mass significantly increases the overall weight of the X-ray tube, and, being located near a high voltage region within the X-ray tube, creates electrical instability.
An alternative approach to noise reduction in X-ray tubes is described in the U.S. Pat. No. 5,253,284 "X-Ray Tube Noise Reduction Using Non-Glass Inserts". Although theoretically satisfactory in certain respects, such X-ray tubes still suffer from significant disadvantages. Thus, by its very nature, the junctions of glass and non-glass materials create manufacturing problems. These problems affect vacuum integrity, radically change the electrical characteristics of the tube, require extensive re-design, and add to the costs of the product.
Yet another conventional approach to noise reduction relies on mechanically isolating the moving parts of the device (rotating target assembly and bearings) from the housing. A typical application of this approach is disclosed in Japanese patent application 60-3898 "X-Ray Tube Device With Rotating Anode". Aside from questions of efficacy, a serious problem with this noise-reduction strategy is that, isolation of the X-ray tube may cause image degradation, since vibrational energy is redistributed, being partially applied to the X-ray tube insert. The quality of output images from computer tomographies and other software-enhanced X-ray diagnostic systems is highly dependent on the focal spot stability.